1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor film that is formed on a substrate having an insulating surface and that has a crystal structure, and to a method of manufacturing a semiconductor device using this semiconductor film as an active layer. More specifically, the present invention relates to a semiconductor device using a crystalline semiconductor film as an active layer and to an electronic apparatus using the semiconductor device as a display unit.
2. Description of the Related Art
In recent years, some have sought advantage in forming a channel forming region from a single crystal, which has less defects, with regard to improvement in mobility of low temperature polycrystalline silicon and to drop in Ioff. A related technology has thus been developed which includes forming an amorphous semiconductor film on a light transmissive substrate with an insulating surface, and crystallizing the film by laser annealing, thermal annealing, etc., to use the obtained crystalline semiconductor film as an active layer of a thin film transistor (hereinafter referred to as TFT).
Laser annealing is known as a crystallizing technology capable of imparting high energy only to an amorphous semiconductor film to crystallize the film. In particular, an excimer laser emitting shortwave light of 400 nm wavelength or less is a representative laser that has been used since early stages of development of laser annealing echnology. In addition to the excimer laser annealing, a technique using YAG laser which is a solid state laser has been developed lately. In the laser annealing mentioned above, a laser beam is processed by an optical system so as to take a spot-like shape or a linear shape on an irradiation surface, and the irradiation surface on a substrate is scanned with the processed laser light (irradiation position of laser light is moved relative to the irradiation surface). For instance, excimer laser annealing using linear laser light is capable of annealing the entire irradiation surface with laser by merely scanning in one direction that is perpendicular to the longitudinal direction of the surface. The excimer laser annealing using linear laser light is thus superior in productivity and is becoming the mainstream in techniques of manufacturing liquid crystal display devices using TFTs. This laser annealing technique has realized a monolithic type liquid crystal display device in which TFTs constituting a pixel portion (pixel TFT) and TFTs constituting driver circuits provided in the periphery of the pixel portion are formed on one glass substrate.
However, the crystalline semiconductor film formed by laser annealing is an accumulation of plural crystal grains, and the crystal grains are positioned at random in the film and the size thereof is irregular. In the TFT fabricated on the glass substrate, the crystalline semiconductor film is divided and formed into an island-like pattern with the intention of partitioning elements. It is therefore impossible to form the TFT with the position and the size of crystal grains specified. The interface of the crystal grains (crystal grain boundary) has a recombination center and a trapping center caused by the amorphous structure, crystal defects, etc., which are factors in degrading the current transportation characteristic of carriers. The potential level in the crystal grain boundary also affects this characteristic.
The crystallinity of a semiconductor film in a channel forming region has a great influence on a TFT characteristic. However, it is almost impossible to form the channel forming region from a single crystal semiconductor film while removing the adverse influence of the crystal grain boundary.
Attempts at growing the crystal grains larger have been made in order to solve this problem. For instance, a method of laser annealing has been reported in xe2x80x9cHigh-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Methodxe2x80x9d, K. Shimizu, O. Sugiwara and M. Matsumura, IEEE Transactions on Electron Devices, vol. 40, No. 1, p.p. 112-117, 1993. According to the method, a three-layer film consisting of Si, SiO2, and Si is formed on the substrate and both sides of the device, i.e., the three-layer film side and the substrate side, are irradiated with excimer laser light. The article states that the method is capable of enlarging the size of the crystal grains by laser light irradiation with a certain energy intensity.
The method proposed by K. Shimizu et al., is characterized in that a thermal characteristic of a base material of an amorphous silicon film is changed locally to control the heat flow to the substrate and to introduce a temperature gradient. In order to introduce the temperature gradient, a three-layer structure consisting of a high melting point metal layer, a silicon oxide layer, and a semiconductor film is formed on a glass substrate. Structurally speaking, to fabricate a top gate type TFT with this semiconductor layer as an active layer is not impossible. However, the silicon oxide film provided between the semiconductor film and the high melting point metal film generates parasitic capacitance and increases power consumption, making it difficult to obtain a TFT operating at a high speed.
Other methods such as a method that uses lasers having a phase difference and the step irradiation method, also have a problem and require a complicated laser apparatus. In addition, when applied to crystallization of driving elements of a liquid crystal panel having a driver circuit incorporated therein, the methods may not always be successful in enabling all part of the channel forming region to have a large grain, nor in crystallizing them into a single crystal, for the elements are usually arranged irregularly, not with regular intervals.
There is another method that is a combination of the dual beam method and the three-layer island structure. (The dual beam method is a method in which an amorphous semiconductor film is crystallized by irradiating each side of a substrate with a laser, or by irradiating one side of a substrate with a laser and then irradiating the other side of the substrate with the laser transmitted through the substrate and reflected by a mirror or the like.) When applied to crystallization of driving elements of a liquid crystal panel having a driver circuit incorporated therein, the combination method is capable of crystallizing a designated site into a single crystal, but is not good at growing a crystal grain to as large a grain size as 5 xcexcm or more. Therefore the method is not suitable for manufacturing a thin film transistor whose channel width is wide. The method also generates a parasitic capacitance between a metal and Si, causing signal delay. Furthermore, the method has a problem of peeling because the temperature sometimes reaches high at a time of irradiation depending on the metal material used.
A method in which a base film is formed from a highly heat conductive insulating film has an advantage in that a parasitic capacitance is not generated between the metal and Si. However, the method requires a development of a highly heat conductive insulating film that is stable.
The present invention discloses techniques for solving these problems. An object of the present invention is to provide a TFT that can operate at a high speed by forming a crystalline semiconductor film while controlling the position and the size of a crystal grain in the film to use the crystalline semiconductor film for a channel forming region of the TFT. Another object of the present invention is to provide a technique of applying this TFT to various semiconductor devices such as a transmission type liquid crystal display device and a display device that uses an electroluminescence material.
In order to attain the objects above, the present invention uses, instead of a metal or a highly heat conductive insulating film, only a conventional insulating film formed on a substrate such as a glass substrate in forming a level difference. The level difference sets a temperature gradient, which is utilized in crystallizing an amorphous semiconductor layer by laser annealing. According to the laser annealing of the present invention, a pulse emitting type or continuous light emitting type excimer laser, YAG laser or argon laser is used as a light source. Laser light emitted from the light source is formed into a linear shape or a rectangular shape by an optical system, and the linear or rectangular laser light is used to irradiate an island-like semiconductor layer. The island-like semiconductor layer is irradiated with the laser from the front side of the substrate (the front side is defined herein as a side where the island-like semiconductor layer is formed), or from both the front side and back side of the substrate (the back side is defined herein as a side opposite to the side where the island-like semiconductor layer is formed).
Following the technique of the present invention, the base insulating film is patterned to form an island-like insulating film, and the level difference caused by this island-like insulating film sets the temperature gradient in carrying out the crystallization. Thermal analysis in this crystallization has been simulated, obtaining results shown in FIG. 5B. The level difference herein designates a convex portion provided in the base insulating film as shown in FIGS. 4A to 4C, or a difference in height between the top (a portion corresponding to a region A in FIG. 1C) and the bottom (a portion corresponding to a region B in FIG. 1C) in an uneven semiconductor film surface caused by the island-like insulating film.
The temperature gradient is supposedly responsible for the results as such. In the region B, heat escapes into 1) a part of the base insulating film right beneath the region B and 2) another part of the base insulating film beside the region B. Therefore, the region B cools faster than other regions. Conversely, a region C receives the heat escaping from the region B and hence is slow to cool down. The temperature gradient is thus generated between the region B and the region C.
Next, an explanation is given of how the semiconductor film is melted completely and then crystallized by the laser light irradiation. Solidification begins first in the region B where the temperature drops most rapidly from the reason mentioned above, and a crystal nuclear is generated. This nuclear serves as the center of crystal growth, and the crystal growth proceeds toward the region C or the region A where the temperature is high and the semiconductor film is in a molten state.
If the semiconductor film is not completely melted by the laser light irradiation and a part thereof remains solid, the solid part (minute solid phase) serves as the center of the crystal growth and the crystal growth proceeds from the center following the temperature gradient. It is thus possible to control the crystal growth so that a crystal having a large grain size is formed in a designated site.
As described above, the base insulating film can be utilized as a heat storage layer or a heat capacity gradient at a desired location and, to do so, forming a highly heat conductive film on the substrate is not necessary. Instead, a structure consisting of a semiconductor film, a base insulating film and a substrate, which has been employed in a conventional TFT formed on a glass substrate, is sufficient. The base insulating film in this structure is patterned to have a desired shape and form the level difference. It is thus possible to control the starting point and the direction of lateral growth by utilizing the temperature distribution in the semiconductor film which corresponds to the arrangement of the level difference.